When three female residents of Baltimore's group home for the mentally impaired are admitted to St. Raphael's Hospital with an unknown virus Dr. Nathaniel McCormick, an investigator from the CDC discovers that something is terribly wrong. The man who had sexual contact with the women is found murdered and Nate fears that the disease he's chasing may not be an act of nature, but of man. Together with Dr. Brooke Michaels an old colleague and former lover they follow the trail of clues to discovery.